Along with the recent development of an information society, a demand has arisen for portable information terminals with processing capability compatible with that of former personal computers. Accordingly, video display devices adapted to higher quality and resolution have been demanded, and thin and light video display devices having a wider viewing angle and low power consumption have been desired. In order to satisfy this demand, efforts have been made to develop methods of manufacturing display devices (displays) comprising thin film active elements arranged in a matrix on a glass substrate (a thin film transistor, a Thin Film Transistor, or simply, TFT).
Most substrates of such display devices wherein an active electrode is formed are fabricated by first forming and patterning a semiconductor film including one or more of amorphous or poly-silicon or the like and then forming metal wire connections thereon. Due to differences in electric characteristics of the active elements, an amorphous silicon display device is characterized in that it requires a driving IC (Integrated Circuit), and a polysilicon display device is characterized in that its driving circuit is formed on a substrate.
Among currently widely used liquid crystal displays (Liquid Crystal Display or simply LCD), amorphous silicon type LCDs are dominant in large liquid crystal displays, while, for medium or small popular liquid crystal displays, polysilicon types, which are suitable for high resolution, are becoming mainstream. As for thin and light self-emissive electroluminescence (organic EL) displays having a wider viewing angle, only the polysilicon type is mass-produced.
Generally, organic EL elements are used in combination with a TFT so that a current flowing thereto can be controlled by utilizing the current voltage control effect of the TFT. “Current voltage control effect” refers to an operation of controlling a current flowing between the source and drain of a TFT, by applying a voltage to the gate terminal of the TFT. With this operation, light emission intensity can be adjusted so that desired gradation can be attained.
However, inclusion of such a TFT-combined structure causes the light emission intensity of the organic EL element to be highly vulnerable to the TFT characteristics. In particular, a relatively large difference is noticed in electric characteristics of the neighboring pixels in the case of a polysilicon TFT, in particular, those which use low temperature polysilicon formed in low temperature processing. The difference is regarded as one factor which deteriorates the display quality, particularly, screen display uniformity, of an organic EL display.
U.S. Pat. No. 6,229,506 discloses a conventional technique for dealing with this problem. Specifically, this document discloses a means for controlling such that the TFT 260, which is originally designed to apply a current drive to an organic EL element 290, flows a gradation current to a data line 220, as shown in FIG. 12.
With this conventional means shown in FIG. 12, a gradation current flowing to the data line 220 is made, through a predetermined procedure, to flow into the driver TFT 260, so that a voltage which is necessary to cause the driver TFT 260 to flow a gradation current into the data line 220 is generated, and a corresponding charge is stored in a holding capacitor 280 (current writing). As the driver TFT 260 continues flowing the gradation current to the organic EL element 290 until next access is attempted, a desired gradation can be attained.
Here, a gradation current to be flowed to the data line 220 is supplied to the data line by a data driver which has a voltage current circuit for receiving RBG video signals and giving voltage-current conversion thereto. When a TFT in the voltage-current conversion circuit is formed in low temperature polysilicon processing, it is difficult to obtain uniform voltage-current conversion characteristics, and non-uniform characteristics cause a problem of deteriorated image quality.